1. Field of the Invention
This invention relates to an automatic positioning and cutting system for use in a float glass production plant and, more particularly, to a system employing a linear stepper motor for automatically positioning a plurality of glass cutting carriages along a bridge which extends transversely across the glass as it exits the forming equipment of the production plant such that the glass can be continuously cut without undue interruptions.
Glass cutting equipment according to this invention provides a more reliable and more accurate means of automatically positioning the glass cutting carriages than previously achieved in prior art systems. In a preferred embodiment, a controller and a linear stepper motor are attached to one side of a carriage to position the carriage, while cutting means are attached to the other side of the carriage to cut the glass material.
2. Description of the Prior Art
To continuously produce flat glass by the "float glass"process, various powdered raw materials, are mixed together and melted in a furnace. This mixture is poured onto the surface of a molten tin or tin alloy bath. The mixture is then passed over the bath to form a sheet of glass. As the semi-molten sheet traverses the length of the bath, it spreads out across the surface area of the bath to form a sheet of glass having plane parallel surfaces and a finite width. The sheet is then allowed to solidify as it progresses along the ever-cooling bath. The solidified glass sheet leaves the tin bath, and is annealed to remove substantially all internal stresses. Cutting to desired lengths and widths is now done. Conventionally the sheet is continuously conveyed to a cutting operation where it is scored longitudinally and then perpendicularly to the direction of travel of the sheet and thereafter broken at the score lines to form panes of the desired dimensions. These panes of predetermined sizes are then conveyed to stations for packaging and shipment.
The present invention relates to improvements in the automation of the first (i.e. longitudinal) scoring step, and represents a significant advance in the art from the point of view of productivity. In order to better understand the significance of this advance, the prior art's problems must be first described.
In general they are as follows: as mentioned earlier, after leaving the float glass forming and annealing apparatus, the cooled glass must be cut (scored) longitudinally an perpendicularly to the direction of travel of the glass sheet to form panes of various predetermined sizes. On average, most plants produce hundreds of tons of glass per day. Clearly, therefore, the cutter (scorer) must be as continuously operated as possible in order to avoid undue waste, and must provide rapid adjustment so that orders for glass panes of different sizes may be filled. When continuity is not achieved, glass not being cut, because, for example, time-consuming adjustments are being made, must be turned into cullet and recycled.
In one prior art cutting method widely employed throughout the float glass industry, the longitudinal or first cut is usually performed by scoring machines having cutters supported by a bridge which extends transversely across and above the sheet of glass such that the cutters can cut the glass longitudinally as the glass moves past the cutters. In order to position the carriage so that the pane size can be varied, several different cutting machines must be utilized because of the continuous glass forming process. In such prior art processes, at any particular time, glass is being cut by cutters on one machine, to accommodate one set of requirements, while cutters on another different machine are being adjusted to accommodate the next order. The setup process, whereby the cutters are reset to accommodate the next order, is inexact, so that the actual widths of the glass strips cut must be measured, and the precise position of the cutters, commonly, are readjusted. The glass cut during the adjustment process obviously cannot be sold and must be turned to cullet. While this recycling to "cullet" is helpful, the output of the production plant is wasted during this time.
It is apparent that this method of cutting glass creates an undesirable abundance of cullet and is time-consuming. Consequently, it would be most desirable to be able to automatically position the cutters to accommodate these varying sizes as quickly and as accurately as possible in order to avoid the rechecking steps.
Exemplary of various prior art attempts to provide semiautomatic cutting systems are the well-known screw-type systems which make use of carriages and the aforementioned plurality of bridges. Alternatively, the systems disclosed in U.S. Pat. Nos. 4,072,887 to Buschmann et al. and 4,170,159 to McNally employ rotary stepper motors and a hybrid system, respectively. Also, a cutting apparatus for moving a cutter head "on the fly" across a piece of glass while employing a linear stepper motor is known, as shown by the parker Hannifin Corporation, Compumotor Division Catalog, 1988. None of these systems achieve true automatic positioning of cutters to accommodate longitudinal scoring (cutting) of glass as it advances off a continuous glass sheet forming operation such as from the float glass process.
Screw type cutting systems, exemplified by the well known Grenzebach screw cutter, generally employ a central screw upon which the cutter carriages are located. In order to maintain the integrity of the system, the screw must be connected to a large beam which serves as the cutter bridge. When it is desired to move the cutter carriages, the screw is rotated causing the carriages to traverse the length of the screw. The carriages are independently positioned with respect to one another using various cumbersome brakes and clutches which interact with the screw. The disadvantages of the screw system are numerous: a low degree of accuracy (plus or minus 0.04 in.), due to the mechanical tolerances between the screw and the brakes and clutches; extended periods of time required to position the carriages and return them to a home position (approximately three minutes required for all carriages to return home); inability to quickly repair and replace a carriage and the screw (approximately eight hours required to replace a screw); high cost; excessive weight of the system due to the large carriages and the massive screw; and the limited number of cutting heads.
Operationally, the most significant problem with the screw system is excessive cullet generated because several pieces of glass must be cut on a trial and error basis each time the carriages are repositioned in order to insure accurate positioning. As this trial and error repositioning step is commonly performed numerous times during the day, the amount of cullet produced can become enormous. Therefore, it would be advantageous to use a glass cutting system that is accurate, quick and avoids the undesirable problems associated with the Grenzebach screw.
The U.S. Pat. No. 4,072,887 to Buschmann et al. discloses a positioning apparatus for cutters in which the carriages are positioned through the use of individually operated rotary stepper motors and controlled by a common controller unit. Positioning of the carriages centers around the positioning of the carriage closest to the home position, and the subsequent positioning of the remaining carriages, with respect to the aforementioned carriage closest to the home position. Unfortunately, rotary stepper motors used this way have some significant disadvantages; namely, mechanical tolerances between the toothed rack and the pinion reduce the positioning accuracy of the device and the inability to quickly repair and replace a cutter.
In U.S. Pat. No. 4,170,159 to McNally, there is disclosed a linear positioning apparatus comprised of cutters attached to carriages, such that the carriages are positioned by a central control means and screw drive mechanism. A motor rotates the screw drive mechanism but the type of motor employed is not disclosed. This apparatus suffers from several problems; for example, the tolerances between the carriages and the screw will affect the accuracy of the system. Moreover, the carriages cannot be replaced or repaired quickly.
While these prior art systems are capable of positioning the carriages, they cannot rapidly position the carriages and return the carriages to a home position, due to the type of positioning means employed. Also, as stated with respect to the Grenzebach screw system, several different cutting machines must be employed in order to semi-continuously cut the glass, as the glass continually leaves the glass forming apparatus. The required switching between the machines results in excess cullet while the necessary adjustments are being made. For example, commonly, 3 -15 minutes are required for the necessary adjustments. Consequently, while the glass forming apparatus is continuously producing glass for 3 -15 minutes, this newly formed glass is not being cut. Instead, this newly formed glass is being turned into cullet. Also, the individual rotary step motors, as taught in Buschmann, et al., do not achieve the optimum accuracy and speed of a linear stepper motor. Finally, these cutters cannot be manually positioned quickly and easily which may become critical if the drive mechanism of the machinery malfunctions.
The flying cutter for cutting various materials disclosed in the Parker Hannifin catalog employs such a linear stepper motor and has several advantages. However in this catalog such motors are used, for example, to cut glass, "on the fly"and only perpendicularly to the direction of travel of the material. In this prior art device, the cutter traverses along the bridge as the cutter makes the perpendicular cut on the fly. The cutter is comprised of a linear stepper motor, a knife, a rail, and a carriage. While this system is highly advantageous, due to the application of the linear step motor, nowhere is it taught or disclosed how to use such motors to accurately position multiple cutters in a stationary position for consistently cutting glass in the longitudinal direction with respect to the continuous movement of the glass, as it emerges, for example, from a float glass operation. It was left to our invention to solve this problem.
While a linear stepper motor is highly advantageous, the inventors have discovered that the required air gap in the linear stepper motor was not being maintained when any appreciable load was applied, thus deterring its use in a continuous longitudinal operation. This is because a linear stepper motor is comprised of a platen and a forcer. The forcer ideally traverses along the platen on a uniform air gap at 0.0005 in. The forcer is connected to a carriage that supports a cutter. The carriage is supported by a rail. As the forcer traverses along the platen, the carriage traverses along the rail. The rail does not contain the same mechanical tolerances as the motor; consequently, there is undesirable play between the rail and the carriage. The inventors determined that this undesirable play resulted in the forcer binding against the platen. Consequently, the cutter could not be automatically or manually positioned with consistency, thus as aforesaid, deterring its use in a continuous longitudinal operation.
It is apparent, from the above, that there exists a need in the art for a float glass cutting and positioning system which rapidly positions the carriages for the longitudinal scoring thereby reducing cullet during production changeover to a different pane of glass size in a continuous glassmaking operation, and which at least equals the durability of the known systems, but which, at the same time overcomes the problems in the prior art devices.
It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan, once given the following disclosure.